The performance.now() method returns a DOMHighResTimeStamp, measured in milliseconds.

The timestamp is not actually high-resolution. To mitigate security threats such as Spectre, browsers currently round the result to varying degrees. (Firefox started rounding to 2 milliseconds in Firefox 59.) Some browsers may also slightly randomize the timestamp. The precision may improve again in future releases; browser developers are still investigating these timing attacks and how best to mitigate them.

Note: This feature is available in Web Workers.

The returned value represents the time elapsed since the time origin.

Bear in mind the following points:

  • In dedicated workers created from a Window context, the value in the worker will be lower than performance.now() in the window who spawned that worker. It used to be the same as t0 of the main context, but this was changed.
  • In shared or service workers, the value in the worker might be higher than that of the main context because that window can be created after those workers.

Syntax

t = performance.now();

Example

var t0 = performance.now();
doSomething();
var t1 = performance.now();
console.log("Call to doSomething took " + (t1 - t0) + " milliseconds.");

Unlike other timing data available to JavaScript (for example Date.now), the timestamps returned by Performance.now() are not limited to one-millisecond resolution. Instead, they represent times as floating-point numbers with up to microsecond precision.

Also unlike Date.now(), the values returned by Performance.now() always increase at a constant rate, independent of the system clock (which might be adjusted manually or skewed by software like NTP). Otherwise, performance.timing.navigationStart + performance.now() will be approximately equal to Date.now().

Reduced time precision

To offer protection against timing attacks and fingerprinting, the precision of performance.now() might get rounded depending on browser settings.
In Firefox, the privacy.reduceTimerPrecision  preference is enabled by default and defaults to 20us in Firefox 59; in 60 it will be 2ms.

// reduced time precision (2ms) in Firefox 60
performance.now();
// 8781416
// 8781814
// 8782206
// ...


// reduced time precision with `privacy.resistFingerprinting` enabled
performance.now();
// 8865400
// 8866200
// 8866700
// ...

In Firefox, you can also enable privacy.resistFingerprinting — this changes the precision to 100ms or the value of privacy.resistFingerprinting.reduceTimerPrecision.microseconds, whichever is larger.

Specifications

Specification Status Comment
High Resolution Time Level 2
The definition of 'performance.now()' in that specification.
Candidate Recommendation Stricter definitions of interfaces and types.
High Resolution Time
The definition of 'performance.now()' in that specification.
Recommendation Initial definition

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support24 Yes151 210158
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support2525 Yes151 2 No9 ?

1. In Firefox 57.0.4 the accuracy was reduced to 20 microseconds.

2. In Firefox 59 the accuracy was reduced to 2 milliseconds.

See also